1. Technical Field
The present invention relates to a sheet supply device for supplying print sheets (print media).
2. Background Arts
Conventionally, known is a sheet supply device that picks up a print sheet from a sheet supply tray and feeds the print sheet to a print unit by its registration rollers. In the sheet supply device, the print sheet is stalled in a state where its leading edge is contacted to a registration nip between the registration rollers to form a slack in the print sheet, so that an oblique feed of the print sheet can be compensated. Then, the registration rollers are driven to feed the print sheet to the print unit.
When the print sheet is fed forward by the registration rollers, the slack of the print sheet is expanded (i.e. back tension is applied to the print sheet) between the registration nip of the registration rollers and rollers located upstream from them. As a result, a noise (sheet expansion noise) may be generated due to the expansion of the slack.
A Japanese Patent Application Laid-Open No. 2010-215389 (Patent Document 1) discloses a sheet supply mechanism that also carries out an assistant operation. In the assistant operation, rollers located upstream from registration rollers are driven while the registration rollers are being driven.
In the sheet supply mechanism, when supplying a print sheet form a sheet supply side tray, the print sheet is picked up by primary sheet supply rollers from the sheet supply side tray, and then stalled in a state where its leading edge is contacted to a registration nip between registration rollers to form a slack in the print sheet. From the state, the assistant operation is carried out by starting to drive the primary sheet supply rollers at the same time as starting to drive the registration rollers.
Specifically, the primary sheet supply rollers and the registration rollers are started to be driven at the same time, and an acceleration of a feed speed applied to the print sheet by the primary sheet supply rollers is made smaller than that applied to the print sheet by the registration rollers. Therefore, a difference between the feed speed by the registration rollers and that by the primary sheet supply rollers increases gradually. As a result, the slack of the print sheet is flattened gradually. After the slack of the print sheet has been flattened, the primary sheet supply rollers are stopped to finish the assistant operation. Since main function of the primary sheet supply rollers is to pick up a print sheet and feed forward the picked-up print sheet, the assistant operation is finished (the primary sheet supply rollers are stopped) before a trailing edge of the picked-up print sheet passes completely through the primary sheet supply rollers in order to prevent the primary sheet supply rollers from picking up the next print sheet erroneously.
By the above-explained assistant operation in the sheet supply mechanism of the Patent Document 1, a back tension applied to the print sheet by expanding the slack of the print sheet is reduced by gradually flattening the slack of the print sheet, compared with a mechanism that carries out no assistant operation.